In some cases, mainly working with multiprogram mpeg-ts containers as
input, it would be handy to select sub stream of a specific program by
their metadata.
This patch makes it possible to narrow the stream selection among
streams of the specified program by stream metadata.

Examples:
p:601:m:language:hun will select all sub streams of program with id 601
where sub streams have metadata key named 'language' with value 'hun'.
p:602:m:guide will select all sub streams of program with id 602 where
sub streams have metadata key named 'guide'.